The unified atomic mass unit, defined as one twelfth the mass of a carbon-12 atom, serves as a standard reference for quantifying the masses of atoms and molecules, facilitating comparisons across the diverse elements of the periodic table.
An atom is made up of protons, neutrons and electrons, with almost all of its mass attributed to the combined masses of the protons and neutrons. Since the relative masses of a proton and a neutron are very similar, it is logical to select a reference isotope whose nucleus has an equal number of protons and neutrons. A few candidates, such as 2H, 4He and 12C immediately come to mind.
Ultimately, 12C was eventually chosen in 1961 because it has an equal number of protons and neutrons – six each – making it a better average for the relative masses of protons and neutrons in the nucleus compared to the other two isotopes. Furthermore, it has a high relative isotopic abundance, which makes it easier to isolate for measurement. Another reason for the choice of 12C is that it was already used as a reference standard in mass spectrometry prior to its selection.
Thus, 12C was arbitrarily assigned a value of exactly twelve unified atomic mass units or 12 u. Everything seems in order after this definition, but how is the carbon-12 unified atomic mass unit scale relevant to a chemist who is more familiar with calculating and measuring the inertia mass of macroscopic amounts of chemicals in the laboratory?
